Stephen D. Morton
and G. Fred Lee University of Wisconsin Madison, 53706
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Calcium Carbonate Equilibria in Lakes
T h e chemistry of the aquatic environment under natural conditions may be quite different from that predicted by idealized laboratory experiments. This article and the following one (1) will discuss some of the problems of the chemistry of lakes and oceans. The calcium carbonate chemistry of lakes and oceans is significant for a variety of reasons. I n some lakes and oceans, calcium carbonate is a large part of the sediments. It is precipitated both by biological processes, such as in clams and many other organisms, and by inorganic processes. The limestone formations of today were formed by calcium carbonate precipitation in the seas of past geologic time. Scale formation in pipes and boilers is governed by the calcium carbonate chemistry of water supplies. Analysis of clamshells, for what is coprecipitated with calcium carbonate, may prove to be a fruitful method of determining past climatological cycles (2). Manv lakes and oceans are thermallv stratified with a warmer, well mixed layer of water near the surface and a colder, poorly mixed layer near the bottom. This thermal stratification hinders complete mixing of the water as contrasted with the easily mixed solutions in reaction vessels in the laboratory. Many lakes lose their stratification in fall, regaining it in spring; the deep water oceans are continually stratified. The figure shows temperature versus depth for Lake n'lendota, Madison, Wisconsin. It can be seen that the thermocline, the region of maximum change of temperature with depth, is nearer the surface in summer than in fall. I n late fall, winter, and early spring, the lake is isothermal from top to bottom. Because of photosynthesis in the upper. layers where light can penetrate, carbon dioxide is consumed and
the p H is higher than near the bot,tom where carhon dioxide is produced by decomposition of dccaying organic matter. However, after fall overturn (disappearance of stratification), the p H is nearly constant from top t o bottom (see the table). Calculation of Calcium Carbonate Solubility
For a nouhydrolyzing substance such as BaSOr, the solubility is given by and (Ra2+) = (SOr2-) in a BaSO.,-H20 system. For a nonhydrolyeing substance of the form AzB3which dissociates int,o two A ions and 6 -three B ions, the solubilit,y = .\/K,,/~OR.~ Calcium carbonate is much more complicated and (Ca2+)is not generally equal to (COs2-) in a CaC08-H20 system. The usual equat,ions are given below wit,h the cquilibrium constants for 25°C. It should be emphasized that K values are temperat,ure dcpendent and that values for t,he appropriate temperature must be used (4-7) in any calculations involving these equations.
.\/KT,
(CsP+)(COJ'-)= KaD= 10-8.J4
(1)
(H+)(HC08c)/(H,C02)= Kt = 10-6.3E
(2)
(H+)(COX1-)/(HCO,-) = Kn = 10-10.53
(3)
+
Note that by convention (H2C0a) means (COX H2COs). The amount of H2C0a species is about 400 times less than t,he amount of dissolved C 0 2 species. Indeed, H,CO3 is act,ually a medium st,rength acid with a pI
